Cytological and ultrastructural comparisons of t¹²/t¹² and normal mouse morulae

Cytological and ultrastructural observations were made of normal (BALB/c) mouse morulae and embryos homozygous for the t 12 mutation, which is lethal at the late morula stage. Normal and t 12 / t 12 embryos are indistinguishable cytologically until the late morula stage, at which time cytoplasmic and nuclear differences become apparent. Different cells within individual normal morulae exhibit differences in cytoplasmic basophilia, and the cells appear closely applied to one another. Differences in cytoplasmic basophilia are not apparent between different cells of t 12 / t 12 morulae, and the cells appear more rounded and less closely applied. Nucleoli of normal morulae are irregular in shape, with projections which often reach the nuclear envelope, while nucleoli of t 12 / t 12 morulae exhibit extreme rounding. Nucleoli of normal embryos exhibit a net‐like nucleolonema, with fibrillar and granular elements, and several pars amorpha. Nucleoli of t 12 / t 12 morulae possess the same structural elements but the nucleolonema appears contracted. Nuclei of mutant embryos often contain several agranular electron‐dense bodies, 0.1–0.7 μ in diameter. These are infrequently observed in normal morulae. Ribosomes exist primarily in clusters in normal morulae, but large numbers of free ribosomes are observed in t 12 / t 12 morulae. Many structures are common to both normal and mutant morulae, such as: fibrous strands, crystalloids, granular endoplasmic reticulum, vacuolated mitochondria, cytoplasmic vesicles, tight junctions, nuclear pores, and intranuclear annulate lamellae. Since the abnormalities exhibited by t 12 / t 12 morulae can be interpreted as degenerative changes, they do not resolve the mode of action of the t locus or the primary cause of t 12 / t 12 lethality.